This invention relates to calorimeters for measuring the heating values of gaseous fuels.
Prior to the 1970s, the cost of pipeline gas and the heat value (BTU content) were more or less constant. Infrequent measurements of BTU content were accomplished by flame calorimeters, while less elaborate and less expensive calorimeters were used for non-critical determinations. With the advent of energy shortages in recent years, there has been a large increase in the cost of natural fuels, as well as a general decrease in the quality of the fuels available. The BTU content of the fuels became more important as suppliers began to mix alternate source gases containing various compositions. As a result, natural gas fuel is now subject to a large variability in heat value and, because of the increased cost, it is becoming more important to charge customers according to the amount of energy which is consumed, rather than solely by the volume of gas which is used.
There are six main methods which have been used to determine the heat value of gaseous fuels: combustion calorimetry, gas chromatography, absorption spectrometry, oxygen titration/flame temperature measurement, mass spectrometry, and gas density.
The standard method used to measure the energy content of natural gas is low temperature combustion calorimetry, in which the gas is burned and the heat produced is transferred to a thermal reservoir, so that the temperature of the product gases rises only a few degrees above the initial temperature. This temperature rise can be measured accurately (to better than 0.001.degree.) and is directly proportional to the energy content of the gas, so that the BTU value of the gas may be readily determined from the temperature increase.
The combustion reaction is typically accomplished at 60.degree. F. so that the reference states of the products are gaseous CO.sub.2 and liquid H.sub.2 O. The advantage of this low temperature approach is that it yields standard states (ASTM conditions), with no systematic uncertainty. Since this method provides a direct measure of the energy content of the gases and the small temperature rise can be measured quite accurately, it is an extremely precise technique.
While low temperature calorimetry has been widely used, it has some inherent difficulties. Most such measurement systems require ambient temperature control or air conditioning, and flow measurements must be accurate. Furthermore, such instruments are generally too large to be used in field applications.
Other methods of calorimetry, such as high temperature calorimetry or catalytic combustion calorimetry, have been used to reduce the expense and increase the response time of the measurement. The combustion method may be accomplished more rapidly by burning the fuel gas in a diluent and measuring the temperature increase, a method known as high temperature or net value calorimetry. The reaction proceeds as in the low temperature approach, but the resulting product is gaseous H.sub.2 O. At higher temperatures, sufficient heat is consumed in raising the temperature of the product gases and vaporizing the water produced to create considerable uncertainty in the actual heat which is referred to standard conditions. A single calibration gas cannot be used effectively, since the amount of water produced varies with the composition of the gas.
Catalytic combustion, rather than direct combustion, can also be used to measure BTU content. In a catalytic measurement system, the gas/air mixture is passed over a catalyst, such as Ni or Pd, causing the oxygen in the mixture to oxidize the hydrocarbon. This reaction produces heat, but the catalyst acts as a heat sink. For continuous measurement, the gas is flowed through a porous catalyst, and an electronic control circuit supplies heat via a heater embedded in the catalyst to maintain the measurement cell at a temperature equal to a reference cell. This method is potentially relatively simple and inexpensive, and the fact that a temperature difference is maintained reduces the uncertainties created by changes in the ambient temperature, pressure, and humidity.
Although a variety of heat measurement techniques are available in the art, a need has developed, particularly with the increase in fuel costs and the decrease in consistency of natural fuel gases, for a measurement technique which is highly accurate without being overly expensive. The high temperature calorimetry techniques known in the art incorporate inherent uncertainties in heating inert gas components and product mixtures and in vaporizing the water produced by the combustion reaction. These uncertainties cannot be completely eliminated unless the composition of the mixture is known. Thus a need has developed for a portable calorimetric technique which can operate at high temperatures, can provide heat values at standard conditions, and is simple and reliable in both the mechanical and electronic aspects of its design.